Hybrid vehicle fuel systems may include a sealed fuel tank configured to withstand high fuel tank pressure and vacuum levels. The vehicle may include a fuel tank isolation valve to seal the fuel tank from the atmosphere. Pressure in the fuel tank may build up due to the generation of fuel vapors. If the pressure inside the fuel tank reaches the capacity of the fuel tank, fuel vapors may be released from the fuel tank into a fuel vapor canister by opening the fuel tank isolation valve. Hydrocarbons (HCs) in the fuel vapors may be adsorbed and stored in the fuel vapor canister, and the rest of the fuel vapors may be vented to atmosphere. At a later time, such as when the engine is in operation, stored HCs in the fuel vapor canister may be purged into an engine intake manifold and combusted as fuel. However, due to non-uniform purge flow within the canister, the fuel vapor canister may not be completely purged. Consequently, retained HCs may breakthrough from the fuel vapor canister and vent to the atmosphere as a bleed emission. A hybrid vehicle may in particular suffer from bleed emissions due to limited engine runtime. Further, bleed emission may be significant for a vehicle that has been parked in high ambient temperature for a long duration.
Other attempts to address bleed emissions including arranging a fuel vapor sensor at the fresh air port of the fuel vapor canister. One example approach is shown by Oemcke et al. in U.S. Pat. No. 6,293,261 B1. Therein, fuel vapor content exiting the fuel vapor canister is monitored in real time by the fuel vapor sensor.
However, the inventors herein have recognized potential issues with such systems. As one example, the fuel vapor sensor needs to be rationalized in the presence of HCs. However, since the fuel vapor sensor is positioned at the fresh air port of the fuel vapor canister, the fuel vapor sensor may only detect HCs when there is HC breakthrough from the canister to the atmosphere. When the fuel vapor canister functions effectively and is thoroughly purged, fuel vapors flowing through the fuel vapor sensor may contain little or no HCs. Due to the sensor's limited exposure to HCs, degradation of the fuel vapor sensor may be left undetected. Consequently, bleed emissions at a later time may not be effectively monitored and controlled.
In one example, the issues described above may be addressed by a method for detecting sensor degradation during fuel vapor canister purge, comprising: sensing fuel vapor vented from a fuel vapor canister to atmosphere via a sensor; and during purging of the fuel vapor canister, actuating valves to flow desorbed hydrocarbons from the fuel vapor canister to the engine, sensing desorbed hydrocarbons with the sensor positioned in the flow path of the desorbed hydrocarbons, and determining sensor degradation based on the sensed desorbed hydrocarbons. In this way, degradation of the sensor may be regularly checked during engine runtime.
As one example, a method for an engine comprises, loading a fuel vapor canister by flowing fuel vapors from a fuel tank to a load port of the canister, and monitoring HC content in fuel vapors vented from a vent port of the canister to atmosphere by a HC sensor. During fuel vapor canister purging, the method flows fresh air first from a purge port to the vent port of the canister, and then flows desorbed HCs to a manifold of the engine via the HC sensor. As the desorbed HCs flowing through the HC sensor, the HC sensor rationality check is performed. After finishing the rationality check, the method flows fresh air from the vent port to the purge port to continue purging the fuel vapor canister. As such, a two-stage fuel vapor canister purging procedure is realized. At the first stage, air flows in a first direction inside the fuel vapor canister during HC sensor rationality check. At the second stage, air flows inside the fuel vapor canister in a second, reversed direction relative to the first direction. The two-stage fuel vapor canister purging ensures that the HC sensor rationality check may be performed frequently in the presence of high concentration of HCs. Further, by purging the fuel vapor canister at two opposite air flow directions, HCs stored in the canister may be thoroughly purged.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.